1. Field of the Invention
The present invention relates to a liquid ejection head ejecting droplets, and in particular, to improvements in the stability of droplets ejected by the liquid ejection head.
2. Description of the Related Art
Many proposed printing apparatuses include ink jet printing apparatuses based on a drop-on-demand scheme. These ink jet printing apparatuses apply kinetic energy to droplets to eject the droplets, which impact a print medium for printing. The ink jet printing apparatuses thus have the advantage of being capable of printing on various print media according to this scheme. The ink jet printing apparatuses further have the advantage of eliminating the need for special processing for fixing ink and allowing high-definition images to be inexpensively obtained. Because of these advantages, the ink jet printing apparatuses based on the drop-on-demand scheme as a printing scheme have been commonly adopted in households and offices as means for outputting image documents. This printing scheme is inexpensively and easily available and is used as printing means for printers, copiers, facsimile machines, and the like, which serve as peripheral apparatuses for computers.
Typical ink ejection methods (ink ejection energy generating elements) for the common ink jet printing scheme include a method using electrothermal conversion elements, for example, heaters, and a method using piezoelectric elements, for example, piezo elements. Any of these methods allows ejection of ink droplets to be controlled according to electric signals. The ink ejection method using electrothermal conversion elements applies a voltage to each of the electrothermal conversion elements to instantaneously boil ink located near the electrothermal conversion element. During the boiling, the phase of the ink changes to rapidly increase a bubbling pressure, allowing ink droplets to be quickly ejected. On the other hand, the ink ejection method using piezoelectric elements applies a voltage to each of the piezoelectric elements to displace the piezoelectric element. During the displacement, a pressure is generated to eject ink droplets. Ejection methods using a print head with electrothermal conversion elements are disclosed in Japanese Patent Laid-Open No. S54-161935 (1979), Japanese Patent Laid-Open No. S61-185455 (1986), Japanese Patent Laid-Open No. S61-249768 (1986), Japanese Patent Laid-Open No. H4-10940 (1992) and Japanese Patent Laid-Open No. H4-10941 (1992).
The ink ejection method using electrothermal conversion elements is more advantageous, in the following point, than the methods utilizing other means such as piezoelectric elements. The former method does not require a large space for installation of elements for printing, enabling nozzles to be integrated together and allowing a reduction in the size of the print head.
To increase the print speed of the ink jet printing apparatus and to further improve image quality, it is necessary to achieve an increase in the number of ink ejections per unit time, a further reduction in the size of ink droplets, and stabilization of the ejection of ink droplets. The number of ink ejections is equal to the driving frequency of a voltage applied to the electrothermal conversion elements. However, the driving frequency decreases consistently with the frequency (hereinafter referred to as a refill frequency) at which ink is refilled from a supply chamber into an ejection port portion and a bubbling chamber.
To allow ink to be continuously ejected, the following operation is performed. After ink is ejected through an ejection port, new ink is refilled into the ejection port portion and the bubbling chamber. The electrothermal conversion elements are then driven again to eject the new ink. At this time, if a long time is required for ink refilling following the ejection of ink droplets, a long time elapses until the next ejection of ink droplets. This makes the printing operation unavailable for a long time, resulting in a long time required for the printing.
Increasing the refill frequency requires a reduction in the flow resistance of the ejection port portion. However, in this case, a simple increase in the diameter of the ejection port increases the size of ejected droplets. This prevents high-definition images from being obtained. This is because the ink jet printing apparatus combines ink droplets in various colors to form an image, so that the size of ink droplets has a close relationship with image quality.
Thus, to improve the ink refill speed in the print head, the print head may be formed such that the ejection port portion has a first ejection port portion and a second ejection port portion provided between the bubbling chamber and the first ejection port portion and having a larger diameter than the first ejection port portion. This enables a reduction in variation in channel width in the ejection port portion and thus in the flow resistance of the ink to ink ejected from the bubbling chamber via the ejection port portion. Thus, the speed at which ink is refilled after the ejection of ink droplets can be increased with the high quality of print images maintained. As a result, the time required for refilling can be reduced.
However, even if the second ejection port portion having the larger diameter than the first ejection port portion is formed between the first ejection port portion and the bubbling chamber to increase the refill speed, the stability of ink ejection from the print head may be inappropriate. The ejection stability as used herein refers to whether the mass or speed of ejected ink droplets or the accuracy of impact on the print medium can be maintained constant even when high quality printing is performed at high speed, that is, even when ink is continuously ejected. There are many possible causes for the instability of ejections. A major cause is meniscus vibration.
After droplets are ejected by the print head for printing, an amount of ink corresponding to the ejection is refilled in the bubbling chamber. At this time, the ink flows into the bubbling chamber and the ejection port portion at a certain velocity. FIG. 21A shows plan view of a nozzle in this condition. FIG. 21B is a sectional view of the nozzle. During ink refilling, the ink is filled into the print head so that the flow velocity of the ink is maximized in a substantially central portion of the ejection port portion as shown in FIG. 21B.
However, upon reaching the ejection port portion, the ink is subjected to the force of the atmospheric pressure and the surface tension acting in a direction opposite to that of the flow. On the ink inside the ejection port portion, an inertia force acts in the direction of the ink flow, whereas the atmospheric pressure and surface tension act in the opposite direction. Thus, during ink filling, vibration (hereinafter referred to as meniscus vibration) occurs around the ejection port surface. If the surface of the ink vibrates during ink ejection, the position of the surface is unstable, and the ink is unstably ejected by the print head. This makes the size of ejected ink droplets unstable and reduces the impact accuracy.
When the ink is ejected while the shape of the ink surface is unstable because of the meniscus vibration, that is, while the surface of the ink is raised or recessed with respect to the ejection port surface, the amount of ink droplets ejected may vary. This may in turn vary the dot diameter of ink droplets, which is an element for formation of images. As a result, the image quality may be degraded.
Furthermore, if the ink flows fast to the ejection port portion and the inertia force of the ink is higher than the atmospheric pressure or the surface tension of the ink itself, the amplitude of the meniscus vibration may increase to cause the ink to overflow the ejection port. The ink may then adhere to the surface of the ejection port, thus reducing the impact accuracy. In this phenomenon, smaller ejected ink droplets are more likely to be affected by the adhering ink. The resulting reduced impact accuracy may degrade the quality of print images.
Therefore, to allow ink droplets to be continuously and stably ejected, the ink is desirably ejected at time intervals such that the meniscus vibration is attenuated sufficiently to stabilize the ink surface. However, if new ink ejection is not started until the meniscus vibration is attenuated to stabilize the ink surface, printing requires a long time, thus reducing the efficiency with which images are formed by the printing.